[platform-ui-dev] Update to General Purpose Navigator design proposal

[Knut Radloff <knut_radloff@xxxxxxxxxx>]

I added an overview and screen shot of the Navigator example to the 
beginning of the design doc as well as made a correction and 
clarification. 
-The INavigatorExtensionSite, not the INavigatorExtension, provides access 
to other navigator extensions. 
-Clarified how/when actions are added by navigator extensions to their 
parent elements in the action contribution section. 
Thanks to Lynne for pointing out these omissions.

The prototype and example code is pretty thoroughly documented. Please ask 
if there are any questions.

I also updated the original M1 RFC document on the UI team home page to 
reflect additional package explorer scenario details and community 
feedback.
All this can be found on the UI team proposals page: 
http://dev.eclipse.org/viewcvs/index.cgi/%7Echeckout%7E/platform-ui-home/docs.html

Knut